Highly recommend Ashford Castle (incredible) and Dromoland Castle (also very nice). Of the two, Ashford is the larger, more castle-like property. The walled garden at Dromoland is absolutely lovely. Dining at both is incredible.
Both castles offer activities, so plan a two-night stay at each (if you have time). We did the Hawk Walk at Ashford, and it was a very unique and enjoyable experience. You can put 'Hawk Walk Ashford Castle' into YouTube and see what it's all about. Both castles have lakes for fishing, etc....Ashford's is larger. Meals at both were top notch. At Ashford, it is an easy walk into the village of Cong.
Previous poster provided the links, and both castles offer multi-night or one-night specials. If there is not one posted for when you are planning to travel, inquire and they will most likely offer you some sort of special.
Can't say enough good things about both properties. We stayed at a combination of high-end castles and modest B&Bs, looking back, that was the perfect mix........allowed us to enjoy the more grass-roots Irish hospitality as well as the hospitality of a five-star castle...............both experiences were special and memorable.
You'll love Ireland. You didn't mention how long you will be in Ireland, but if you have plenty of time, don't miss the Rock of Cashel, the town of Kinsale, the Dingle, Connemera, Connor's Pass, etc. So much to see.